document max version of Python

Pelican depends on wsgiref, which fails to install with Python 3.3.
According to wsgiref package presentation[1], it does not support Python >= 3.2.

[1] https://pypi.python.org/pypi/wsgiref/0.1.2
use less ambiguous form for Python version restrictions
Merge pull request #9 from duthils/python-max-3.2

document max version of Python